Self-Assembled Bismuth Selenide (Bi2se3) Quantum Dots Grown by Molecular Beam Epitaxy

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Self-assembled Bismuth selenide (Bi2se3) quantum dots grown by molecular beam epitaxy

Marcel S. Claro CUNY City College

Ido Levy CUNY City College

Abhinandan Gangopadhyay Arizona State University

David J. Smith Arizona State University

Maria C. Tamargo CUNY City College

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OPEN Self-assembled Bismuth Selenide (Bi2Se3) quantum dots grown by molecular beam epitaxy Received: 5 September 2018 Marcel S. Claro 1,5, Ido Levy1,2, Abhinandan Gangopadhyay3, David J. Smith4 & Accepted: 28 January 2019 Maria C. Tamargo1,2 Published: xx xx xxxx We report the growth of self-assembled Bi2Se3 quantum dots (QDs) by molecular beam epitaxy on GaAs substrates using the droplet epitaxy technique. The QD formation occurs after anneal of Bismuth droplets under Selenium fux. Characterization by atomic force microscopy, scanning electron microscopy, X-ray difraction, high-resolution transmission electron microscopy and X-ray refectance spectroscopy is presented. Raman spectra confrm the QD quality. The quantum dots are crystalline, with hexagonal shape, and have average dimensions of 12-nm height (12 quintuple layers) and 46-nm width, and a density of 8.5 × 109 cm−2. This droplet growth technique provides a means to produce topological insulator QDs in a reproducible and controllable way, providing convenient access to a promising quantum material with singular spin properties.

Te electronic confnement in all three dimensions of semiconductor quantum dots (QDs) leads to unique quantum properties. A discrete energy spectrum is produced, and the confnement impacts how the electrons interact with each other and to external infuences, such as electric and magnetic felds. Tese quantum efects can be tuned by changes in the sizes of the dots or the strength of the confning potential. Such QDs are used in many diverse applications, spanning from devices such as lasers1, solar cells2 and photodetectors3, to the study of new physical phenomena, such as single photon interactions4 and spin manipulation5. In the well-known common semiconductors, QDs are typically created using heterostructures defined by lithography or by self-assembled crystal growth. QD growths by molecular beam epitaxy (MBE) in Stranski-Krastanov mode6 and by droplet epitaxy7,8 are the most commonly used techniques for Si-Ge, and III-V semiconductors. QDs could also be formed by the electrostatic confnement of 2D electron gases9. However, in materials where the electronic states are protected by time-reversal symmetry, i.e., Graphene10, and the class of materials known as Topological Insulators (TI), the electrostatic potential cannot confne or scatter electrons as usual, a property known as the Klein paradox11. Tus, quantum confnement in these materials can normally only be achieved by the formation of 0D nanostructures. Tree-dimensional TIs, such as Bi2Se3 and related materials, are insulators in the bulk form, usually with a narrow band gap. However, they have surface states with spins that are locked with momentum and protected by time-reversal symmetry12. Tin flms possess favorable bulk to surface volume ratio to enhance the TI properties and have applications that include quantum computing, dissipation-less electronics, spintronics, enhanced ther- 12 moelectric efects and high performance fexible photonic devices . Among the many TIs, Bi2Se3 is particularly interesting because its band gap is larger than those of most other TIs, and the experimentally verifed Dirac cone is at the gamma point13. Tese materials exhibit a tetradymite crystal structure with Se-Bi-Se-Bi-Se units, 14 commonly referred to as quintuple layers, that are bonded together by van der Waals forces . Bi2Se3 can be synthetized as bulk crystals, thin flms or nanoparticles by several methods15. Bulk crystals have been synthetized by Bridgman–Stockbarger16, fux17 and other methods, and then chemically or mechanically exfoliated to study their properties as thin flms18. Excellent results have been achieved this way. Unfortunately, these methods have poor controllability and low yield, and cannot be applied for large-scale production or large-area applications. Solution-based processes have also been applied successfully to synthesize nanoparticles and to study their novel

1Department of Chemistry, The City College of New York, New York, NY, 10031, USA. 2Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New York, NY, 10016, USA. 3School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ, 85287, USA. 4Department of Physics, Arizona State University, Tempe, AZ, 85287, USA. 5INL – International Iberian Nanotechnology Laboratory, 4715-330, Braga, Portugal. Correspondence and requests for materials should be addressed to M.C.T. (email:[email protected] )

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Figure 1. Spotty (1 × 1) RHEED pattern observed afer formation of Bi2Se3 QDs. Te inset illustrates the steps involved in the growth sequence.

optical properties, from photothermic absorption19 to nonlinear optical properties such as saturable absorbers, which have applications in photonics20. Te chemical vapor deposition (CVD) method can produce relatively high quality nanomaterials on a large scale15. However, none of these methods can achieve the purity and controllability of MBE14. In MBE the material grows in ultra-high vacuum using only fuxes of high purity Bi and 21 Se. High quality Bi2Se3 has been grown successfully by MBE on diferent substrates and, due to the ability to controllably dope materials achieved with MBE, the Quantum Anomalous Hall efect could be observed22. Additionally, MBE-grown QDs can be readily incorporated into multilayers and heterostructures with other TIs23 as well as with 3D materials24, thus greatly expanding the range of possibilities for novel physical phenomena and device applications. As with other materials, we expect that some properties of TIs and Bi2Se3, especially those related to spintronic and quantum computing, can be enhanced by quantum dot confnement25–27. In lithographically defned QDs, the quantum confnement was already previously demonstrated28. However, to our knowledge, very few experiments have been done to investigate the efect of the quantum confnement of TIs. Te lack of adequate means to fabricate mesoscopic structures in a reproducible, high purity and controlled way is identifed as a major obstacle. 29 Nonetheless, the MBE growth of Bi2Se3 occurs by van der Waals epitaxy since it is a layered material, and strain cannot be used to induce QD formation by the Stranski-Krastanov mode as in other materials. In this work we demonstrate a viable method to create self-assembled quantum dots of Bi2Se3 by molecular beam epitaxy based on the droplet epitaxy technique. Results and Discussion Te samples were grown in a dual-chamber Riber 2300P system equipped with in situ refection high-energy electron difraction (RHEED). Te GaAs semi-insulating (001) substrates were prepared by oxide desorption in the frst chamber, followed by the growth of 200 nm of GaAs at 575 °C by conventional MBE. Te substrates were cooled under an overpressure of Arsenic and a fat c(4 × 4) surface reconstruction, typical of As-terminated GaAs, was observed by RHEED at the end of this process. Te substrates were then moved under ultra-high vacuum (UHV) to the second chamber for growth of the Bi droplets and the QDs. Te background pressure of the second chamber, where the droplet and QD growth occurred, was 1–4 × 10−10 Torr during growth. High-purity 6N bismuth (Bi) and selenium (Se) were provided by a RIBER double-zone cell for Bi and a Riber VCOR cracker cell for Se. Fluxes were measured by an ion gauge placed in the path of the fuxes. Te estimated fux ratio was 10:1 Se to Bi. Te temperature used for the Bi source was 730 °C and the beam equivalent fux (BEP) was 2 × 10−8 Torr. Tese conditions led to an average growth rate of 32 nm/h for Bi2Se3 by van der Waals epitaxy. Te growth of the bismuth droplets was initiated by opening the Bi shutter when the prepared substrate temperature reached 250 °C, and the fat c(4 × 4) surface reconstruction was still visible. During the frst few minutes, a circular ring started to appear in the RHEED pattern and the streaky pattern from GaAs began to fade, indicating the formation of amorphous material on the surface. Te RHEED pattern became totally difuse afer 15 min with barely visible spots that appeared in some orientations. For the droplet sample, the Bi shutter was closed at that time and the sample was cooled down and extracted for analysis. For QD formation, afer the Bi shutter closure, the Se shutter was opened and the sample was exposed to Se fux at the same temperature of 250 °C. Te RHEED pattern changed from the difuse pattern to a “spotty” (1 × 1) reconstruction pattern. Afer approx. 2 mins, the RHEED pattern was totally transformed into the pattern shown in Fig. 1. Te droplets were exposed to the Se fux for 40 min in total to ensure the complete formation of the Bi2Se3 and to minimize Se vacancies. However, since the RHEED was stable afer the frst few minutes of growth, it should be possible to reduce this time considerably in future studies. Te inset in Fig. 1 illustrates the steps that occur during the QD growth.

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Figure 2. AFM images: (a) bismuth droplets; (b) Bi2Se3 QDs; (c) SEM image of Bi2Se3 QDs. All pictures have the same scale.

Figure 3. (a) High-resolution X-Ray difraction of Bi2Se3 QDs. X-ray refection (XRR) is shown in the inset. (b) High-resolution X-Ray difraction pattern of Bi2Se3 QDs capped with 100 nm of ZnSe. Te inset shows 1 µm × 1 µm AFM image of the surface.

Figure 2a shows an atomic force microscope (AFM) image of the Bi droplets. Tey have semi-spherical shape, with average height of 29 nm and density of 4 × 109 cm−2. Despite diferences in the growth conditions, these results are comparable with Bi droplet growth reported in the literature30. From the AFM image, (Fig. 2b), the QDs are measured to have an average height of 12 nm (12 quintuple-layers) and an average diameter of 46 nm, and the QD density is ~8.5 × 109 cm−2. Te size achieved is sufcient to pre- serve the characteristics of 3D TIs while, at the same time, some confnement is expected according to theoretical predictions25 and previous measurements28. Te transition to TI in thin flms occurs at a thickness greater than about 6 quintuple layers31. In comparison to the radius, height, density, and fnally the total volume of the Bi droplets, the QDs are smaller, shorter and of higher density. Moreover, the QD density is approximately twice the droplet density. Several factors could explain this diference, such as the presence of some Bi desorption during the annealing process, ripening of droplets during QD formation, and changes in the Bi droplet size due to surface migration of Bi during cooling. Tese efects were observed in self-organized growth of nanostructures in other materials32. Due to convolution of the probe tip and the QDs, the QD shape is blurred in AFM images. However, from SEM images, as shown in Fig. 2c, the facets and the hexagonal shapes of the QDs are still preserved. Notably, AFM images repeated afer samples were stored at room temperature, in ambient atmosphere, for over nine months, were unchanged from the original ones taken immediately afer growth, indicating that the QDs are stable. Based on previous results for Bi-droplet formation on GaAs surfaces30, it is anticipated that the QD size can be varied in a controlled and reproducible manner by adjusting the growth parameters, such as Bi deposition temperature and time. We also note that due to the absence of strain as the driving mechanism for QD forma- tion33, defects such as stacking faults, which appear in Stranski-Krastanov QDs, are not expected in QDs made by the droplet epitaxy technique, making this approach highly desirable. To establish the QD stoichiometry, the composition at the surface was measured using energy dispersive x-ray (EDX) analysis coupled to an SEM. Despite strong substrate signals from Ga and As, values of 2.3% of Bi and 3.9% of Se were found. Te Se peak overlaps with the As peak causing small deviations in the Se content. Taking this into account, the ratio of Bi to Se from these measurements indicates the correct approximate stoichiometry for Bi2Se3. High-resolution X-ray difraction (HR-XRD) and X-ray refection (XRR) experiments were also performed on the QDs. Te results, afer the alignment in the substrate (004) peak, are presented in Fig. 3a. Te most intense Bi2Se3 peaks: (0003), (0006) and (00015) can be identifed in the 2θ-ω scan. Tese demonstrate the presence of

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Figure 4. High-resolution scanning transmission electron micrograph recorded using large-angle bright-feld imaging mode, showing Bi2Se3 QDs capped with 100 nm of ZnSe. Arrow indicates top of GaAs substrate.

crystalline Bi2Se3 with the (0001) plane aligned to the (001) plane of the substrate. Te noisiness and broadness of the peaks are not surprising due to the small amount of Bi2Se3 present on the surface and the inherent roughness of the QD layer. Te roughness is also confrmed by the rapid damping observed in the measured XRR curve (Fig. 3a inset). Since the GaAs substrate surface remains exposed, any epitaxial material compatible with GaAs could be grown by MBE to cap the QDs. Te capping layer will primarily protect the Bi2Se3 QDs from exfoliation and oxidation, and it could also serve as a barrier for carrier confnement. Unfortunately, Bi2Se3 will desorb from the surface when the sample temperature is raised above 300 °C. Tus, further growth of GaAs could not be used for capping purposes. Zinc Selenide, ZnSe, a wide-gap semiconductor (Eg = 2.7 eV), has a very similar lattice parameter as GaAs and it can be grown as a strained layer on GaAs34. ZnSe has also been previously grown over 35 Bi2Se3 layers . Te ideal temperature for ZnSe epitaxial growth is close to 250 °C, which is the temperature used for the QD growth. Based on these considerations, another sample was grown where the QDs were capped with 100 nm of ZnSe. Tis growth took place in the same chamber, immediately afer the QD growth. Te zinc was provided by a RIBER Zn Knudsen efusion cell while the Se fux from the VCOR cell was maintained. Te growth rate of ZnSe was 200 nm/h. Te sample was exposed to Zn and Se fuxes for a period of time such that the overgrown layer thickness, assuming fat-layer growth, was ~100 nm. Te HR-XRD pattern for the overgrown sample is shown in Fig. 3b. Te Bi2Se3 peaks are still present as before, and narrow peaks corresponding to the (002) and (004) refections of ZnSe can be observed adjacent to the corresponding GaAs peaks. Te lattice mismatch relative to the GaAs is −0.57%, which means that the ZnSe layer should grow epitaxially and pseudomorphic to the GaAs substrate. From the AFM analysis, which is shown in the inset of Fig. 3b, it is notable that the surface is still rough even afer growth of the capping layer (RMS smoothness ~7.4 nm) due to the initial roughness originating from the QDs, as well as ZnSe grains and defects. Furthermore, another peak is also visible in the HR-XRD pattern. Tis peak is attributed to the growth of grains of wurtzite ZnSe or of misoriented ZnSe on top of the Bi2Se3 QDs by van der Waals epitaxy36. Chen36 and Hernandez-Mainet35 et al., described the preferential growth of the wurtzite (hexagonal) phase when ZnCdSe was grown on Bi2Se3. However, we were unable to fnd the same described peaks or symmetry in the HR-XRD pattern. Instead, the peaks and symmetry observed indicate the presence of (111)-oriented ZnSe grains. Aberration-corrected electron microscopy was used to image cross sections of the ZnSe-capped Bi2Se3 QDs: examples are shown in Figs 4 and 5. It is clear from Fig. 4 that the interface between GaAs and ZnSe is abrupt and continuous with few defects. Bismuth atomic columns appear with high contrast in both high-angle annular-dark-feld (HAADF) and large-angle bright-feld (LABF) image modes. Tus, the layered crystal (quintuple layer) structure of the Bi2Se3 QDs is readily identifed in Fig. 4. Moreover, one can also clearly see the columnar growth of misoriented ZnSe, with stacking faults in the capping layer originating from the QDs or grain boundaries. Figure 5a,d clearly show the Bi2Se3 layered structure at very high resolution. Te atomic stacking visible in the ZnSe capping layer directly above the Bi2Se3 QD in Fig. 5a suggests that ZnSe in this region grows as a disordered mixture of (0001) wurtzite and (111) zincblende structures, as previously observed in ZnSe nanobelt growth37. Fast Fourier Transforms (FFTs) of the TEM micrograph from areas above the QDs (Fig. 5c), on the sides of the QDs (i.e., on bare GaAs) (Fig. 5b), are very similar, despite the rotation, supporting the presence of tilted and defective zincblende ZnSe rather than the disordered wurtzite phase that is visible above the QDs. Finally, it is interesting that this image also suggests the presence of some bismuth at the grain boundaries, and the QD height is slightly greater close to the QD edge than in the QD center. Tis implies the occurrence of some Bi segregation during the capping process, similar to what has been reported for Indium in the InAs SK QDs38. Te Raman spectra of both samples is presented in Fig. 6. In the QD sample, the surface is mostly transparent and the GaAs LO (292 cm−1) substrate peak is the most intense, which is not the case in continuous flms since the surface is refective in the visible range due to the metallic behavior of the flm. Two of the main peaks of bulk −1 Bi2Se3 can be noted at 174 and 131 cm , and they correspond to the A1g and Eg modes, respectively. Tese peaks are very similar in the capped and uncapped samples, and the only diference is the addition of the ZnSe LO peak −1 (254 cm ) from the cap layer. Te Raman spectra shown in Fig. 6 confrm the high quality of the Bi2Se3 QDs. Te

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Figure 5. Aberration-corrected electron micrograph of Bi2Se3 QDs capped with 100 nm of ZnSe: (a) High- angle annular-dark-feld (HAADF) image. (b) FFT of GaAs region in (a). (c) FFT of ZnSe region in (a) rotated by 45°. (d) Detail of quintuple-layer stacking visible inside the Bi2Se3 QD. Inverted contrast large-angle bright- feld (LABF) image.

Figure 6. Room temperature Raman spectra of Bi2Se3 QDs uncapped and capped with ZnSe.

positions of the Eg and A1g modes do not present the blue-shif or the change in the intensity ratio that has been observed when the thickness is reduced to a few quintuple layers, and Bi2Se3 loses the TI properties (less than 6 quintuple layers)39 consistent with the observed QD thickness.

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Conclusion Te droplet epitaxy technique has been successfully applied to the growth of quantum dots of Bi2Se3, which is a layered van der Waals material that is not amenable to the more conventional strain-driven Stranski-Krastanov techniques of self-assembly via MBE. Te Bi2Se3 QDs can be grown with high density and they have good crystal quality and electronic properties. Both uncapped and ZnSe-capped Bi2Se3 QDs were demonstrated. Te size achieved is opportune, since electron confnement for a 3D Topological insulator would be anticipated. Te QD formation process, as with other self-assembled techniques, is simple, reproducible and tunable, and should be ideal for the fabrication of large-area devices and experiments wherein the signal of several comparable quantum dots is required. Te results ofer the means to explore new physics and device applications possible with these unique low dimensional structures. Methods Te HR-XRD measurements were performed using a Bruker D8 Discover 3D difractometer with a da Vinci con- fguration, 1-mm Slit/Collimator, and a Cu Kα1(1.5418 Å) source. Te AFM images were captured with a Bruker Dimension FastScan AFM with a FastScan-A silicon probe and the SEM images with a FEI Helios Nanolab 660. Te aberration-corrected imaging was carried out with a JEOL ARM-200F scanning transmission electron microscope operated at 200 keV. Te probe convergence angle was set at 20 mrad, and the image collection angles were 0–22 mrad for large-angle bright-feld imaging and 90–150 mrad for high-angle annular-dark-feld imaging. Te Raman spectra were performed on a WiTec confocal microscope using a 1 mW 532 nm laser focused by a 50x lens. Data Availability Te datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. References 1. Bimberg, D. & Pohl, U. W. Quantum dots: Promises and accomplishments. Mater. 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